Dog kidney cortex contains two cytoplasmic thyronine-binding proteins (CTBPs) of molecular weight 50,000-70,000 ("big" CTBP) and 6,000 daltons ("little" CTBP), respectively. Both moieties reversibly bind thyroxine (T4) and triiodothyronine (T3). CTBP is recoverable from dog kidney cells maintained in culture. Studies in progress are designed to define the function(s) of CTBPs in cultured kidney cells. Dog myocardial cytoplasm contains two proteins (A, B) which nondissociably bind up to 50% of tracer T4 and T3. These proteins differ from CTBPs in terms of molecular weight and displaceable binding of hormones. Current studies analyze the biochemical factors which influence the incorporation of T4 and T3 into these cytoplasmic moieties. Effects of thyroid hormone and of insulin on cultured kidney cell cation transport are under investigation, and the physical characteristics of renal membrane insulin receptors have been defined.